Answer:
From Wikipedia:
Pius XII died on 9 October 1958 of acute heart failure brought on by a sudden myocardial infarction in Castel Gandolfo, the Papal summer residence. His doctor Gaspanini said afterwards: "The Holy Father did not die because of any specific illness. He was completely exhausted. He was overworked beyond limit. His heart was healthy, his lungs were good. He could have lived another 20 years, had he spared himself."